Grist has, alongside its environmental policy news and commentary, a running feature called “Ask Umbra” in which people ask for advice on ecologically responsible consumption. The answer almost invariably turns out to be “this hinges on an impossibly complicated set of considerations.” For example, is it better to buy frozen vegetables or steamed ones:
Grade A frozen foods are harvested when ripe and quickly taken to the freezing plant, where they are (even more quickly) flash frozen at extremely low temperatures. The modern industrial freezing process retains almost all the original nutritional value of the food (according to nutrition guru Marion Nestle’s helpful book What to Eat). Good to go on the nutrition angle. But it’s important to have an efficient freezer. One study using 1970s data found that the longer frozen foods sit in the freezer, i.e., are using energy in storage, the more they fall behind canned goods in the efficiency smackdown.
The canned goods are a bit less nutritious, but a study that looked closely at this issue found the differences between frozen and canned carrots to be insignificant. Carrots in syrup, or whatever they might put carrots in, would of course fall in to the category of dessert or a processed food, and cannot be favorably compared to fresh. As you know, the ecological issue with canned carrots is the steel can itself, which has high embodied energy costs. If a study assumes the recycling of the steel can, then canned vegetables can compete favorably with frozen vegetables on the sustainability index.
From a political perspective, this sort of thing underscores the need for collective action in the form of public policy that will put a price on greenhouse gas pollution. To realistically assess the total environmental impact of the choice between frozen carrots and canned carrots, you’d also want to know something about the land-use impact of your decisions, the transportation of the goods, the energy costs of keeping frozen food frozen in the supermarket, etc. You can’t really do this sort of thing through back-of-the-envelop calculations.